I have 30,000+ photos synced to iCloud and am trying to get their thumbnails to show on the Photos app on iOS 8.3. On the (first) Photos tab, viewing at Years or Collections levels shows lots of grey placeholders instead of photos. Only when I click into Moments do the thumbnails actually load.
However, trying to scroll through all my photos at the Moments level, to trigger downloading of the thumbnail, would take hours. (It's a little faster under Albums > All Photos, but not much.)
Is there a way to force all the thumbnails to download? I tried disabling iCloud Photo Library and re-enabling it, but it did not force a sync. Note that I have Optimize iPhone Storage set, but I thought this should still download thumbnails.